Weight losses for specimens of aluminum alloys exposed for one year to moderately severe industrial and marine atmospheres were similar for groups of specimens put out initially in winter or summer. The depth of pitting on those put out in the winter was, however, from 1½ to 2½ times as great as on those put out in the summer. The difference is explained by the specimens exposed in winter remaining covered with a film of moisture for a greater proportion of the time during their first few weeks or months on test. It is recommended that, when comparing the atmospheric corrosion resistance of aluminum alloys, tests should be started both in summer and in winter.